Minimize SQL round trips

Question

We want to minimize SQL roun trips:
1. We have 3 object types: parcel, folder and document
     object schema:
          parcel - contains one or more folders          folder - contains one or
more documents
document - belong to one folder
     db schema contains these as 3 tables.
     use case: show all documents belong to all folders of a parcel.
     objective: minimize number of SQLs issued. Want to avoid[minimize]:
          1. one SQL to get collection of folders
          2. iterate thru folder collection, and issue separate SQL to get
documents belong to one folder.
If we have 50 folders, we will issue 51 SQL statements.
     possibility:
          Is there a way to be able to performa a "join" of folder and document to
obtain the collection via one SQL?? mapping? extension?

Answer 1

Stephen,
Just to clarify, here is simple example.
Did you mean something like this?
public class Person {
long id;
String name;
List addresses=new ArrayList();
public long getId() {
return id;
public void setId(long id) {
this.id = id;
public String getName() {
return name;
public void setName(String name) {
this.name = name;
public List getAddresses() {
return addresses;
public void setAddresses(List addresses) {
this.addresses = addresses;
public class Address {
long id;
String street;
Person person;
public long getId() {
return id;
public void setId(long id) {
this.id = id;
public String getStreet() {
return street;
public void setStreet(String street) {
this.street = street;
public Person getPerson() {
return person;
public void setPerson(Person person) {
this.person = person;
Query q = pm.newQuery();
q.setCandidates(pm.getExtent(Person.class,true));
q.setFilter("id>0");
List persons = (List)q.execute();
q=pm.newQuery();
q.setCandidates(pm.getExtent(Address.class, true));
q.declareParameters("java.util.Collection persons");
q.declareVariables("Person p");
q.setFilter("persons.contains(p) && p==person");
outputPersonsByIteratingOverAddressList((List)q.execute(persons));
when I am iterating over list of Addresses and doing .getPerson() there is
no additional select statement to load Person, so in this way it works,
but it would not work if I would be iterating over Person list and doing
getAddresses(), then additional select statement is executed per every
call regardless, what I kind of can understand (jdo needs to know which
Address instances belongs to this Person...)
Rihards
Stephen Kim wrote:
As I recall, in 2.5+ you may be able specify a Collection parameter in
which you could pass the document instances:
pm.newQuery (Folder.class, "paramFolders.contains (folder)")
Though I haven't tested this at all and may not work.
Rihards Freimanis wrote:
Abe White wrote:
Kodo 3.0 will support the type of "eager" loading of related objects you
are looking for. In the meantime, though, you may be able to meet your
objective with a single JDOQL query.Abe,
How JDOQL would help in this situation?
The problem is in way FCO are loaded, once I have collection of documents
and I am trying to access folder fields for that document, for every
document in collection additional SQL statement to load folder (assuming
that every document is in different folder) is executed.
Stephen Kim
[email protected]
SolarMetric, Inc.
http://www.solarmetric.com
S